This invention relates to magnesium-based alloys and in particular to magnesium-based alloy electrodes for use in batteries.
Magnesium has long been used as the anode in dry batteries in combination with a carbon-manganese dioxide cathode mix. An electrolyte used in one variety of these dry batteries comprises magnesium perchlorate, although alkali and alkaline earth bromides are often used. Small amounts of soluble chromates may be added to the electrolyte to reduce wasteful corrosion of the anode.
Alloys of magnesium have also been used in dry battery anodes. These include aluminum and zinc as alloying elements, the former having a beneficial effect on apparent current efficiency and the latter reducing the time lapse of delayed action i.e. the time lapse before the anode's protective film is disrupted and full operating voltage is achieved. Typical examples of these alloys include Mg Az31 and Az61, as designated by ASTM.
Since magnesium crystallizes in the hexagonal crystal system its usefulness is limited due to lack of cold rollability and cold-forming characteristics. These disadvantages also apply to the aforementioned magnesium-aluminum-zinc alloys.
It is known that magnesium-lithium alloys are partly or wholly crystallized in the cubic crystal system (depending upon the composition) characteristic of pure lithium, and hence are workable by straight forward metallurgical techniques. Thus, the addition of lithium to magnesium in sufficient quantities can change the crystal structure of the resultant alloy from hexagonal (magnesium) to cubic (lithium). These alloys are relatively soft and, for a Mg-base alloy, uniquely ductile. The cold forming troubles associated with the hexagonal crystal structure of magnesium and magnesium rich solid solutions may thus be avoided. In addition, lithium has a lower density i.e. 0.534 g/cc, than magnesium and hence, the addition of lithium to magnesium decreases the density of the resultant alloy making it lighter than pure magnesium.
According to the Mg-Li equilibrium phase diagram Freeth W. E. and Raynor, G. V., J. Inst. Metals 82 575-80 (1954), the solid .alpha. phase i.e. hexagonal crystal structure characteristic of pure magnesium exists up to about 5.7%/w lithium in the alloy.
Above about 10.3%/w lithium, the solid .beta. phase i.e. cubic crystal structure characteristic of pure lithium takes over. Between these limits a mixed solid solution (.alpha. and .beta.) exists.